When the heart of a patient is examined using an MRI apparatus or other imaging apparatus, it is important to eliminate motion artifacts caused by the rapid motion of the heart. To date, the image acquisition of the MRI is synchronized to the moment of the patient's heartbeat using an external trigger signal. It is customary in the art to derive such a trigger signal from an electrocardiogram that is acquired simultaneously with the MRI imaging. The high magnetic and electromagnetic fields involved in the MRI imaging superimpose a rather high level of interference onto the weak electrical signals that have been picked up from the skin of the patient. In addition, electrolytes in the blood of the patient represent moving charges that generate even more interference in combination with the high magnetic field. This interference is termed magneto-hydrodynamic effect. Both types of interference deteriorate the detection of the heartbeat to the point where it can no longer serve as a reliable trigger signal for the MRI imaging. This causes the motion artifacts, which the triggering with the heartbeat was intended to avoid, to reappear in the resulting MRI images. In addition, when the heart of an unborn fetus is to be imaged using MRI, the skin of the fetus is not accessible for picking up an electrocardiogram, so no trigger signal for the MRI imaging is available at all.
To overcome these drawbacks certain publications (e.g., see, F. Kording, B. Schoennagel, G. Lund, F. Ueberle, C. Jung, G. Adam, J. Yamamura, “Doppler Ultrasound Compared with Electrocardiogram and Pulse Oximetry Cardiac Triggering: A Pilot Study”, Magnetic Resonance in Medicine, doi: 10.1002/mm.25502 (2014)) propose, and demonstrate an MR-compatible cardiotocograph, CTG, to acquire the heartbeat of adult patients during MRI. The heart of the patient is examined with ultrasonic waves, and the reflected ultrasonic waves that are Doppler-shifted by the motion of the heart are detected. From this signal, the exact moment of the heartbeat is extracted using a wavelet-based peak detection. (See, e.g., J. Yamamura, I. Kopp, M. Frisch, R. Fischer, K. Valett, K. Hecher, G. Adam, U. Wedegartner, “Cardiac MRI of the Fetal Heart Using a Novel Triggering Method: Initial Results in an Animal Model”, Journal of Magnetic Resonance Imaging 35, 1071-1076 (2012)) propose, and demonstrate on sheep, MRI imaging of a fetal heart that is triggered by a CTG signal. The Doppler frequency shift is proportional to the speed of movement of the heart; therefore, the Doppler-shifted ultrasonic signal can serve as a suitable triggering signal for the image acquisition in the MRI:
When the heart of a fetus is to be imaged using MRI, a high level of noise is superimposed on the CTG signal by the blood flow, breathing of the mother and by the MRI. As a robust trigger signal has to be processed in real-time without false negative or false positive trigger detections, there is therefore a need for an improved ultrasonic detection of the heartbeat during MRI.